In recent years, a water-absorbent resin is widely utilized in sanitary materials (absorbent articles) (e.g. disposable diapers, sanitary napkins, incontinent pads) and as their main component material in order to cause the water-absorbent resin to absorb body fluids (e.g. urine, blood).
Known examples of the above water-absorbent resin include: partially-neutralized and crosslinked poly(acrylic acids); hydrolyzed graft polymers of starch-acrylonitrile; neutralized graft polymers of starch-acrylic acid; saponified copolymers of vinyl acetate-acrylic acid esters; crosslinked carboxymethyl cellulose; hydrolyzed copolymers of acrylonitrile or acrylamide, or crosslinked polymers of these hydrolyzed copolymers; crosslinked polymers of cationic monomers; crosslinked copolymers of isobutylene-maleic acid; and crosslinked copolymers of 2-acrylamido-2-methylpropanesulfonic acid and acrylic acid.
As water absorption properties which the above water-absorbent resin is desired to have, there have hitherto been preached such as, on occasions of contact with aqueous liquids (e.g. body fluids), high water absorption capacity, excellent absorption rate, liquid permeability, gel strength of swollen gel, suction amount of sucking up water from base materials containing the aqueous liquids.
In recent years, as to the sanitary materials (e.g. disposable diapers), their high functionalization and thinning are making progress. Thus, the thinning is aimed at while the increase of the absorption amount and the prevention of the leakage are aimed at by increasing the amount (g) and ratio (weight %/ratio in absorbent structure) of the water-absorbent resin being used. The sanitary materials in which the water-absorbent resin has been increased as mentioned above are on the favorable track from the viewpoint of simply storing a liquid. However, in practical use for diapers, the water-absorbent resin has had problems of, due to water absorption, swelling to thus form a soft gel, thereby causing what is called gel blocking in the diapers wherein the gel blocking causes the absorption amount reduction and the leakage.
Thus, the liquid permeability of the water-absorbent resin is taken notice of in recent years. For example, water-absorbent resins enhanced in the liquid permeability are reported (e.g. refer to patent documents 1 to 7 below). However, if attempts are made to cause prior water-absorbent resins to exercise high liquid permeability, then the enhancement of the liquid permeability needs the enlargement of particle diameters and the extension of spaces in the gel, so the water-absorbent resins which have high liquid permeability have generally had problems of deteriorating in the liquid-sticking-up property.
It has also been known that the particle size distribution contributes very much to the liquid permeability, and arts of controlling the particle size of the water-absorbent resin have also been known (e.g. refer to patent documents 8 to 11 below), but have had problems that the liquid-sucking-up property deteriorates if the particle size is enlarged. The liquid-sucking-up property is an important property having been known so far (e.g. refer to patent documents 12 and 13 below), but it has been very difficult to obtain a particulate water-absorbent resin composition enhanced in both of the “liquid permeability” and “liquid-sucking-up property” of the water-absorbent resin, because both are antithetical physical properties.
Incidentally, the “liquid permeability” in the present invention refers to the liquid permeation performance after the particulate water-absorbent resin composition has absorbed water under load, that is, liquid permeability between swollen gel particles under load, and is a liquid permeability model in diapers during practical use. In addition, the “liquid-sucking-up property” in the present invention refers to the liquid-sucking-up or liquid-distributing performance when a water-unabsorbed particulate water-absorbent resin composition absorbs water, that is, a rate at which water-unabsorbed dry particles suck up a liquid or a rate at which a liquid is distributed into the particulate water-absorbent resin composition, and is a liquid distribution model (which has not been found out until the present invention is made) in diapers during practical use.                [Patent Document 1] Pamphlet of WO 95/26209        [Patent Document 2] EP 0 951 913 B        [Patent Document 3] EP 0 640 330 B        [Patent Document 4] Pamphlet of WO 2001/066056        [Patent Document 5] Pamphlet of WO 98/47454        [Patent Document 6] U.S. Pat. No. 6,414,214        [Patent Document 7] US Patent Publication No. 2002/128618        [Patent Document 8] U.S. Pat. No. 5,051,259        [Patent Document 9] EP 0 349 240 B        [Patent Document 10] EP 0 579 764 B        [Patent Document 11] EP 0 629 411 B        [Patent Document 12] EP 0 532 002 B        [Patent Document 13] U.S. Pat. No. 6,399,668        